To date, a variety of ultrasonic motors using the piezoelectric effects have been proposed. Patent Literature 1 below discloses an ultrasonic motor including a stator in which a discoid piezoelectric element is attached to one surface of a discoid oscillator. FIGS. 8(a) and 8(b) are respectively an outline front sectional view illustrating the ultrasonic motor described in Patent Literature 1 and a schematic plan view of the oscillator explaining a polarization structure of the piezoelectric element on a lower surface.
In an ultrasonic motor 101 that is disclosed in Patent Literature 1, two B (1, 3) mode standing waves that are out of phase with each other by 90° are generated, and a travelling wave is generated by combining the two standing waves.
The ultrasonic motor 101 includes a support plate 102. A central shaft 103 is attached at the center of the support plate 102. The central shaft 103 extends upward from the center of the support plate 102. A stator 104 is secured to the central shaft 103, thereby holding the stator 104 with the central shaft 103 and the support plate 102.
The stator 104 includes a discoid oscillator 105 and a discoid piezoelectric element 106 that is attached to a lower surface of the oscillator 105. As illustrated in FIG. 8(b), the piezoelectric element 106 is divided into 12 sector areas each having a central angle of 30°. Each sector area is polarized in the thickness direction as indicated by “+” or “−” signs in the figure. The sector areas indicated as “+” and the sector areas indicated as “−” are polarized in opposite thickness directions. In order to obtain an n-wave travelling wave (n is a natural number) by generating B (1, n) mode or B (0, n) mode standing waves, the piezoelectric body needs to be divided into 4n areas. Here, n is a natural number. Thus, in Patent Literature 1, the piezoelectric element 106 is divided into 12 areas in order to obtain a three-wave travelling wave.
The piezoelectric element 106 includes electrodes formed on two opposite surfaces of a polarized piezoelectric ceramic plate. By applying an alternating voltage to the piezoelectric element 106, areas indicated by the “+” sign and areas indicated by the “−” sign as described above oscillate with phases opposite to each other, thereby oscillating the oscillator 105 attached to the piezoelectric element 106. With this oscillation, displacement is repeated between an oscillation mode indicated by dotted line A and an oscillation mode indicated by dotted line B in FIG. 8(a), and, B (1, 3) mode standing waves are generated. By driving the piezoelectric element 106 so as to generate two standing waves that are out of phase with each other by 90°, a travelling wave having a diameter smaller than the oscillator 105 is generated.
A protrusion 105a are formed on an upper surface of the oscillator 105 in a radial direction in which the travelling wave propagates, and an unshown rotor is disposed on an upper surface of the oscillator 105 so as to contact the protrusion 105a. The rotor is supported by the central shaft 103 so as to be a rotating member rotating about the central shaft 103. Thus, by generating the above-described travelling wave, the rotor contacted by the protrusion 105a is rotated about the central shaft 103.
Non Patent Literature 1 below discloses ultrasonic motors using the B (1, n) mode including the above-described B (1, 3) mode and the B (0, n) mode.
Patent Literature 1: Japanese Unexamined Patent Application Publication No. 7-194151
Non Patent Literature 1: Japan Technology Transfer Association, Solid Element Actuator Kenkyu Bukai (Solid Element Actuator Research Working Group) Eds.; “Kakudai Kino Wo Yusuru Disc-gata Cho-onpa Motor (Disc-type ultrasonic motor having enhancement features)”; Handbook of New Actuators for Precise Positioning; Fuji Techno Systems; 839-841.
In the ultrasonic motor 101 described in Patent Literature 1, the rotor is rotated due to generation of the travelling wave generated by combining two B (1, 3) mode standing waves that are out of phase with each other by 90°. Although rotation efficiency in this case is comparatively high, the substantially discoid piezoelectric element 106 needs to be divided into 12 areas and each area needs to be polarized in order to realize the ultrasonic motor 101. For this reason, a forming process and a polarizing process of the piezoelectric body is complex, and formation of the electrodes also needs to be complex. Thus, the cost of the ultrasonic motor cannot be reduced.